The present invention relates to demodulators and, more particularly, to stereo signal demodulators for demodulating FM (Frequency Modulated) composite stereo signals of pilot tone systems.
An FM composite stereo signal of a pilot tone system, is generally described as a composite signal consisting of a main channel signal, a sub-channel signal, a pilot signal and a SCA (Subsidiary Communication Authorization) signal. The main channel signal is a summation signal (L+R) of left and right audio signals, and the sub-channel signal contains a component of a difference signal (L-R). The sub-channel signal is an AM (Amplitude Modulated) signal of a subcarrier signal (38 KHz) modulated by the difference signal. The pilot signal is a 19 KHz signal and is a reference signal used for the separation of the left and right audio signals. The SCA signal is used for auxiliary communication. However, this SCA signal is a signal component which is not necessary for the stereo demodulation. It is generally removed from the stereo composite signal before the composite signal is supplied to the stereo demodulator. Therefore, the term, "the stereo composite signal" means, hereinafter, the composite of main and sub-channel signals and the pilot signal.
The circuit for separately extracting the right and left signals from this composite signal are mainly two systems, one being a switching-type circuit and the other being a matrix-type circuit.
In the matrix-type circuit, after a filter separates the stereo composite signal into the main channel signal and the sub-channel signal, the sub-channel signal is demodulated by the subcarrier signal of 38 KHz to produce the difference signal (L-R). Those signals (L+R) and (L-R) are summed and subtracted to recover the signals L and R. However, the matrix-type circuit is not used so commonly because the circuit construction is complicated and the operation stability of this type is poor.
According to the switching system, on the other hand, the composite signal is switched so that it is separated into two signals L and R. Since the circuit construction of the switching system is simple and since the operations are relatively stable, the switching system has recently been used almost exclusively.
Although various types of circuits are proposed and used as a demodulator of the switching type, a demodulator using a differential amplifier, as disclosed in U.S. Pat. No. 3,617,641, is generally used because it is easily made in the form of a semiconductor integrated circuit. That is, the composite signal is fed to the common emitter junction of two transistors constituting the differential amplifier. They are separated by supplying a subcarrier frequency signal of 38 KHz to the base of one transistor. The other subcarrier frequency signal has a phase which is opposite to the above subcarrier signal and is supplied to the base of the other transistor.
In this instance, the separated left and right signals have the opposite signal components superposed thereon, more or less, as crosstalk components. A circuit for cancelling those crosstalk signal components is generally added. Thus crosstalk cancelling circuit is designed to attenuate the composite signals so that they have substantially the same signal level as the crosstalk components. The attenuated composite signals are separated into the attenuated left and right signals. The attenuated left and right signals of the separated left and right signals are added to cancel the crosstalk signal components. An example of such a crosstalk cancelling circuit is also described in the above U.S. patent as a circuit of resistors 100, 101 and 102 and transistors 60, 71 and 72.
The attenuation of the composite signal is usually performed by means of a T-type resistor circuit. However, in a stereo demodulator of the switching type, having such crosstalk cancelling circuit, both the attenuation of the composite signals and the separation of the attenuated composite signal are performed by a cascade connected circuit of the T-type resistor circuit; two differential amplifier type switches, and a load resistor. Especially, the T-type resistor circuit is inserted between the emitters of two transistors of the lower positioning different differential amplifier. As a result, the emitters of those transistors have a preset DC potential which is determined by the T-type resistor circuit. Consequently, bias potentials applied to the differential amplifiers, connected in series, have to be set to consider the DC potential, and the active elements, (such as the respective transistors) operate in a linear range. Even if the points described above are considered to set the bias potentials, together with the electric characteristics (such as the distortion factor), a the demodulator having the conventional crosstalk cancelling circuit cannot operate with good characteristics if power supply voltage drops significantly.
A description is now given of the case in which the power supply voltage is reduced or drops significantly. Since a DC voltage loss caused by the T-type resistor circuit is inevitable, the bias voltages applied to the active elements (such as transistors) are accordingly lowered. Thus, the active elements then begin operate in their non-linear regions. In the worst case, the transistors are driven into their saturated regions, and in the signal injecting operation for cancelling the crosstalk components is not accomplished. Then, the separation factor of the demodulation signals deteriorates remarkably. Even if the active elements are operated at a higher supply voltage which is free from a reduction of the separation factor, the margin of the voltage supplied to the active elements is lowered by the DC potential loss of the T-type resistor circuit so that the distorsion factor characteristics of the separated stereo signals deteriorate.